1. Field of the Invention
Among general information communication equipment, electroacoustic equipment, measuring equipment, and systems for handling an analog acoustic signal, the invention relates to an input and an output of those equipments or systems. More particularly, the invention relates to an electroacoustic transducer of a digital type which is used for a combination of an analog acoustic signal and a digitized equipment or system.
2. Description of the Related Art
Hitherto, for a combination of an acoustic signal as an analog signal and a digital equipment or system, a microphone of the analog type and an analog/digital converter are generally used additionally on the input side and a digital/analog converter and a loudspeaker or earphones of the analog type are used additionally on the output side. According to such a system, not only special electronic equipment such as analog/digital converter and digital/analog converter are necessary but also an electronic circuit, equipment, and parts adapted to both of the analog and digital types are necessary. Consequently, there are drawbacks such as increase in costs, decrease in reliability, increase in electric power consumption, and the like many items which are difficult to be technically solved such as a generation of noises due to the mixture existence of an analog signal and a digital signal and the like.
As one of the examples devised to solve the above drawbacks, a loudspeaker of a piezoelectric type which is directly driven by a digital signal has been disclosed in a document (Takesaburo Yanagisawa, "Present Existing State of Loudspeaker Directly Driven with Digital Signals", magazines of The Institute of Electronics Information and Communication Engineers of Japan, Vol. 78, No. 5, pp. 565-569, June, 1995). As schematically shown in FIGS. 10A and 10B, an electrode of the loudspeaker of the piezoelectric type is radially divided and each area (angle) is made to correspond to the position of each bit digit of a binary digital signal. FIG. 10A is a cross sectional view of an almost circular loudspeaker. FIG. 10B is a diagram showing a structure of a driving electrode on a piezoelectric diaphragm. In FIGS. 10A and 10B, reference numeral 1 denotes a piezoelectric diaphragm; 2 a stainless sheet; 3 an aluminum sheet; 4 an aluminum ring; and 5 driving electrodes divided and insulated by linear radial boundary lines 6.
However, according to the system of the loudspeaker of the piezoelectric type with such a construction, the boundaries which are divided and insulated are linear radial and are matched with the node and antinode of the natural split vibrating mode of the vibrator, namely, circular diaphragm, so that steep concave and convex portions are caused on frequency characteristics. In the example, to suppress the steep concave and convex portions, a device to attach a stainless sheet or aluminum ring with high rigidity onto a circumference or the like is made. There are, however, problems such that a structure becomes complicated, a weight of the vibrator increases, an efficiency deteriorates, and the like.
Although a digital electric signal can be converted into an analog acoustic signal under such conditions, the analog acoustic signal cannot be converted into the digital electric signal. Even if the equipment or the like is constructed by using an apparatus like the above example, therefore, since an analog signal is handled in an input, there are problems such that the problems of the noise due to the mixture existence of the analog and digital signals and the like still remain.
Also, digital microphones have been disclosed in some documents.
JP-A-2-272998 discloses a digital microphone including a diaphragm, a vibration transmitting member for transmitting vibration of the diaphragm, a conductive slider attached to a part of the vibration transmitting member and conductive patterns arranged with intervals therebetween and for touching the conductive slider along its vibrating locus. However, this digital microphone has a mechanical contact, and therefore, electroacoustic characteristics would be reduced.
JP-A-4-167798 discloses a digital microphone including a piezoelectric substrate having each surface on which a plurality of electrodes having different areas from each other are formed. An electric current value flowing from each of the electrodes, which is proportional to an area of the electrode, is compared with a threshold value so as to generate binary data. However, it is well known that a microphone of a piezoelectric type has inferior electroacoustic characteristics to that of a microphone of a condenser type.
JP-A-7-23492 discloses a digital microphone including a conductive diaphragm, a fixed electrode and an oscillator for generating an FM (frequency modulation) signal having a frequency in accordance with a capacitance formed by the conductive diaphragm and the fixed electrode. A number of pulses output from the oscillator is counted for a period of a sampling frequency and a difference between the number of pulses and reference value is calculated so as to output digital audio data. However, it is believed that this microphone is not a digital microphone but an analog microphone because the frequency modulation is an analog technique.